1. Field of the Invention
The present invention relates to an apparatus and a method for discriminating the information transmitted using a radio signal, and more specifically to an apparatus and a method for discriminating the information transmitted from a base station in a mobile communications system.
2. Description of the Related Art
Some methods have conventionally be suggested or developed as procedures of commencing communications in the mobile communications system. For example, a random access control is adopted in the W-CDMA (Wideband CDMA) of the IMT-2000 (International Mobile Telecommunication 2000).
In the random access control, advertisement information is broadcast from a base station to each mobile station in the communications area of the base station. Upon receipt of the advertisement information, the mobile station transmits a connection request to the base station when it requests to start communications. The connection request includes a terminal identifier of a predetermined bit length referred to as a “signature”. The “signature” to be transmitted is selected at random by the mobile station from among a plurality of predetermined codes, and a transmission timing of it is selected at random by the mobile station from among a plurality of predetermined probable timings.
Upon receipt of a connection request including the “signature”, the base station determines whether or not the requested communications are to be enabled, and the determination result is transmitted to the mobile station. At this time, the determination result is transmitted to the mobile station using a channel referred to as an AICH (Acquisition Indicator channel). Practically, when a base station enables requested communications, a received “signature” is returned as is to the mobile station using a predetermined slot (hereinafter referred to as a “notification slot”) in the AICH. On the other hand, when requested communications are to be disabled, the notification slot is set in a no-signal state.
The mobile station demodulates the AICH, and determines whether or not the “signature” is set in the notification slot. If the “signature” is set in the notification slot of the AICH, then the mobile station determines that the requested communications are enabled, and starts the communications. On the other hand, if no “signature” exists, the mobile station determines that the requested communications have been disabled, and stop the subsequent processes.
The above mentioned discrimination procedure includes a process of multiplying the code stored in the notification slot of the AICH by the “signature”, a process of integrating the multiplication result, and a process of determining whether or not communications are enabled based on the integral. When the communication is enabled, the integral become a “predetermined value Sc” corresponding to the power or amplitude of a received signal. If the communication is disabled, the integral is “0”. The mobile station detects whether or not the communications have been enabled based on the integral.
In the wireless communications system using phase modulation, information is transmitted normally using a predetermined signal point on a phase plane. The reception device regenerates the information by detecting the signal point of a received wave.
However, the position of the signal point in the phase plane fluctuates by noise. Therefore, the position of the signal point detected in the reception device fluctuates at random relative to the position of the signal point set in the transmission device, and the distribution of it is Gaussian. When the position of the signal point of a received signal follows the Gaussian distribution, the integral in the above mentioned determining process also follows the Gaussian distribution. Therefore, when the requested communications are enabled, the integral indicates the Gaussian distribution based on the “Sc” as shown in FIG. 1A. That is to say, when the requested communications are enabled, the integral is the most probably “Sc”, but can be any value in the range between “Ls” and “Hs”. On the other hand, when the requested communications are disabled, the integral indicates the Gaussian distribution based on zero. That is to say, when the requested communications are disabled, the integral is the most probably zero, but can be any value in the range between “Lo” and “Ho”.
Therefore, the mobile station can determine whether or not the requested communications have been enabled by comparing a computed integral with a predetermined threshold as shown in FIG. 1A. Practically, the mobile station determines that the requested communications have been enabled if the integral is larger than the threshold. If the integral is smaller than the threshold, the mobile station determines that the requested communications have been disabled.
However, if the communications environment between the base station and the mobile station is not good and has much noise, then the distribution of the integral is extended as shown in FIG. 1B, and the integrals overlap each other. If the distance between the base station and the mobile station is long and the receiving power is small, then the integral obtained when the communications are enabled is small as shown in FIG. 1C. Then, the distributions of the integrals overlap each other. Therefore, if a threshold for determination as to whether or not the requested communications have been enabled is not appropriately set, then the mobile station may mistakenly determine that they are disabled although the base station enables the communications, or also may mistakenly determine that they have been enabled although the base station disables the communications.
For example, assume that the threshold TH1 is set in the communications environment in which the distribution as shown in FIG. 2A is obtained. In this case, if the computed integral is larger than the threshold TH1 although the requested communications have been disabled, then it is mistakenly determined that the communications have been enabled. At this time, the mobile station assumes that the communications have been enabled and performs corresponding processes although the requested communications have been disabled. Here, the radio signal transmitted from the mobile station in this processes is an interference signal with other channels. Mistakenly determining that the communications have been enabled although the requested communications have been actually disabled is hereinafter referred to as “misdetection”, and the probability that it is mistakenly determined that the communications have been enabled although the requested communications have been actually disabled is referred to as a “probability of misdetection”. The probability of misdetection is represented by the ratio of the area of the shaded portion to the area enclosed by the Gaussian distribution curve shown in FIG. 2A.
Otherwise, it is assumed that the threshold TH2 is set in the communications environment in which the distribution shown in FIG. 2B is obtained. In this case, if the computed integral is smaller than the threshold TH2 although the requested communications is enabled, then it is mistakenly determined that the communications have been disabled. At this time, the mobile station assumes that the communications have been disabled although the requested communications have been enabled, and stops the corresponding processes. That is to say, although a connection could have been made to the base station, the opportunity is lost. Mistakenly determining that the communications have been disabled although the requested communications have been enabled is hereinafter referred to as “omitted detection”. The probability that it is determined that the communications have been disabled although the requested communications have been enabled is hereinafter referred to as a “probability of omitted detection”. The probability of omitted detection is represented by the ratio of the area of the shaded portion to the area enclosed by the Gaussian distribution curve in FIG. 2B. The probability that it is correctly determined that the communications have been enabled when the requested communications have been enabled is referred to as a “probability of correct detection”. The “probability of correct detection” is represented by “1—probability of omitted detection”.
In the existing system, the threshold for use in determining whether or not the requested communications have been enabled is not uniquely defined, but is set such that the above mentioned probability of misdetection can maintain a constant value. Practically, it is assumed that the threshold is a value proportional to the root of the variance of a received signal. And using this threshold, the probability that disabled communications are mistakenly started can be suppressed at or lower than a predetermined value. This threshold is described in, for example, R4-010593 “Correction of AICH performance” of TSG-RAN Working group 4 (Radio).
However, if the threshold is determined as described above, the following problems occur.
(1) When the communications environment is good, the distribution of the integral obtained when the requested communications are enabled and the distribution of the integral obtained when they are disabled do not overlap each other, or overlap a little. Therefore, if the threshold is appropriately set as shown in FIG. 1A, the probability of misdetection and the probability of omitted detection can be zero or substantially zero. However, if the threshold is set such that the probability of misdetection is maintain a constant value, then the misdetection can occur at a certain rate although the misdetection can be avoided.
(2) When the communications environment is not good, the distribution of the integral obtained when the requested communications are enabled and the distribution of the integral obtained when they are disabled overlap each other to a large extent. In this case, if the probability of misdetection maintains a constant value, the probability of omitted detection is increased. That is, although the start of the communications is enabled, there is an increasing number of cases in which an opportunity of connecting to the base station will be lost.
(3) If the variance of a received signal is not correctly computed, there can be a stronger probability of misdetection. If the misdetection occurs, the communications not enabled by the base station can be started. Therefore, in this case, noise increases in the paths on which communications are being established.
Thus, in the conventional system, the threshold for determination as to whether or not the requested communications have been enabled has not always been determined in the optimum method. As a result, there can be the case in which no connection can be made to the base station although the communications are enabled, or the case in which the communications are started although they are not disabled.
The problem is not limited to the AICH, but can also occur with other control signal transmitted from a base station to a mobile station.